See if this works for the objects you want. This is stovak's reply from another thread:

'Configure / Control Keys' under 'Mouse Move Object 3D'

Assign a key for 'Z-Move'

To change the height of an object (artillery or stationary armour), select the object, go into 3D mode (Enter) and hold the Z-Move key while moving the mouse.

And there’s this from the Readme for 4.14:

- Objects from the "Artillery" and "Stationary Armor" sections have added the ability to set the height of the object above ground level in the Full Mission Builder. The properties of such objects also have the ability to specify their location in the event of their destruction: "Do not change the height of an object", "Drop an object to the surface", "Hide an object under the surface".